1. Technical Field
The present invention relates to keyboard switches and keyboard assemblies. More particularly, the present invention relates to an improved piezoelectric keyboard switch and keyboard assembly, including the circuitry for encoding key-entered data.
2. Discussion of the Prior Art
Conventional data entry keyboards generally employ single pole switches configured in an X-Y matrix array which is scanned electronically in order to determine the actuation (i.e. key-up) or non-actuation (i.e. key-down) status of individual keys. Key status data is then encoded by a microprocessor (for example, to ASCII code), buffered and then forwarded serially to a data terminal.
In X-Y matrix keyboards of the type described, seven to ten millisecond keyboard scan cycle periods are employed because individual key bounce periods range between two and ten milliseconds. As a result, nearly simultaneous depression of two or more keys within a time aperture of less than ten milliseconds may result in an out-of-order sequence character printout, unless the scan sequence happens to be in the same order in which the keys were depressed. For example, assume the characters "t" and "h" are stroked consecutively in a period less than the total keyboard scan period, and that the scan sequence position for "t" is after the scan sequence position for "h". The "h" would be displayed before the "t". Alternatively, a complex and costly priority encoder could be implemented to overcome this problem in the X-Y array concept.
As data transfer rates between keyboards and terminals become higher, more stringent requirements will be made of keyboard hardware beyond the viable limit of X-Y matrix technology. A possible solution to this problem is the use of parallel key encoding by means of an array of uniquely coded piezoelectric switches which apply their binary coded signals to parallel data buses. Such an arrangement is described in U.S. Pat. No. 3,940,637 (Ohigashi et al). The keyboard switch structure disclosed in the Ohigashi et al patent employs a keybar which is selectively depressed to resiliently deform a film layer transversely of the plane of that layer. The film has a plurality of conductive circuits or lines disposed thereon in a parallel serpentine design to serve as the data buses. Individual areas of the film are disposed below respective keybars, each area including a plurality of piezoelectrictreated portions of the film. The piezoelectric portions are connected to respective data buses in accordance with a binary code which uniquely defines each key switch. When a key is depressed, its actuator bar deforms the film in its area to generate electrical signals in the piezoelectric portions of that area. Those signals appear on the data buses to which the deformed piezoelectric portions are permenantly connected.
While the Ohigashi et al keyboard arrangement eliminates many of the problems inherent in the X-Y matrix arrays, it unfortunately introduces some of its own disadvantages. Specifically, location of multiple switch areas on the same film can result in incidental actuation of surrounding switches when the film is deformed by a single actuator bar. Moreover, the resilience of the deformed film results in some key bounce problems as well as unfamiliar "feel" and "sound" characteristics upon depression of a key by the operator.
Apart from the characteristics of the key switch itself, the Ohigashi et al patent does not concern itself with the practical requirements of circuitry required to convert the parallel binary key codes into serial data required for terminal entry. Such keyboard entry circuitry must not only convert the character data to serial form, it must also provide for such functions as "character repeat", "shift", and other such functions in a reliable and inexpensive manner while preventing interference between successively entered characters.